In the past, a prepreg has been formed by impregnating a fabric substrate with a resin composition containing thermosetting resin and heating thus-obtained substrate until the resin composition is in a semi-cured state (see, Patent Literatures 1 to 3). Besides, a metal-clad laminate may be formed by disposing metal foil on the prepreg formed in the aforementioned manner, and a printed wiring board may be formed by patterning the metal-clad laminate to make a patterned conductor. Thereafter, a package (PKG) is prepared by mounting a semiconductor element on the printed wiring board and enclosing the element.
Recently, a PoP (Package on Package) has been used for a smartphone and a tablet PC. The PoP includes a plurality of stacked sub-packages. Therefore, mounting performance of the sub-packages and connection reliability between the sub-packages are important. The mounting performance and the connection reliability are improved with a decrease in an absolute value of a warpage of the package (including sub-package) at a room temperature and with a decrease in an amount of change in the warpage caused by a change in the ambient temperature from the room temperature to 260° C. Therefore, substrate material for reducing the warpage of the package has been vigorously developed.